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HMRC internal manual

Oils Technical Manual

Flow meters - other information: Other types of flow meter

Type of flowmeter and description What the meter measures Main usage and strengths Weaknesses
Ultrasonic – use sound waves – there are two types, the cheaper doppler and the more accurate transit time. The ‘doppler’ meter measures the frequency of the reflected wave and the ‘transit time’ meters the time difference between a wave going upstream and one sent downstream. Both meters infer velocity of the flow and can give readings from a static start. The meter does not obstruct flow. Not all fluids conduct ultrasonic waves and opaque fluids degrade the signal, as do fluids that coat the pipe wall. Dirt coating the transducers in the meter will also degrade accuracy, as will any turbulence.
Vortex shedding and fluidic (Coanda effect) The vortices caused by an obstruction in a vortex meter or the oscillations in a fluidic meter, all of which increase with flow. Can be used in pipes from .025 inch to over 12 inches diameter. (Sold for use in petrochemical industries). Inaccurate at low flow rates, affected by vibration and can be affected by compressed pipe work.
Thermal – heat is applied to a sensor, and a second heat probe measures how much heat is left. The faster the flow the less heat is picked up by the second sensor. Commonly applied to gases: the meter does not have to be compensated for gas density. Fluids that coat the sensor or fluids that may vary in their thermal properties.
Variable area – a vane or a float allows more fluid to pass it the faster the flow. The faster the flow the more the movement Used in petrochemical industries. Used to give local indication of small flows. Dirty, and opaque fluids, and fluids that coat surfaces can all affect measurement.
Magnetic resonance – fluids that conduct electricity move through a magnetic field in the pipe that generates a voltage. The faster the flow the more the voltage. Does not obstruct pipes. Commonly used for water- containing products and can be used for corrosive products, and in chemical processes. Should not be used near its limit. Needs a conductive liquid.